You can remove the mulch but don't bother with the soil - removing the soil will likely do more harm to the tree than the weed killer (if that in fact was the problem). Just water the tree surrounds thoroughly to flush the pesticide down into the soil beyond the roots. I'd suggest you seriously reconsider the use of weed and feed products. Not only are they a notoriously inefficient and expensive way to deal with lawn weeds but they are a major contributor to water pollution (including drinking water). And if you have pets or small children, they can be harmful as well. And many W&F products contain atrazine, which is documented to have an adverse affect on live oaks, although it will fall far short of killing them. Just use the lawn fertilizer of your choice judiciously (personally, I'd prefer to go organic) and then spot treat for any weeds or you can remove them manually. And whatever you do, never "dump" left over fertilizers or pesticides. Dispose of them responsibly via whatever hazardous waste disposal programs your area offers. Finally, a 'lawn guy' spraying weed killer in a flowerbed is a recipe for disaster. If it managed to drift far enough to do foliar damage to the oak, think what it might be doing to the other plants in that bed. Obviously the needed care to apply the product safely and according to label directions was lacking. Either ask him to remove the weeds from the bed manually or do it yourself - pulling weeds can be rather therapeutic....you kind of get into this zen-like state :-) Then mulch the cleared out bed well to keep the weeds from returning.

That is VERY impressive. Think of it as the wood cells losing track of which way they are supposed to grow. There was likely a wound there and the encapsulation process went a little crazy. Would I be correct it is a structural problem?

It could just be stressed from repotting, many trees take a turn for the worse after then perk back up after a few weeks. My first Meyer looked like that and I severely over watered it. It was my lack of experience with citrus (it looked sad so I kept watering it) and using a really water retentive soil mix (miracle grow potting soil) that eventually did it in. You totally can successfully grow in potting soil but you have to make sure it dries out between watering's. I can't see the whole size of the tree but in a more water retentive mix you generally will want a smaller pot than usual to combat how much water is being kept near the roots, bigger pot = more wet area for the roots to sit in and if the root system isn't big enough to utilize all the water quickly it will stay wet for prolonged periods of time and potentially rot. I assume you watered just before taking the picture and that is why the soil looks so wet but when you go to water again, stick your finger 2-3 inches into the soil half way between the trunk and the edge of the pot. If it still feels damp hold off, you want it to be almost dry without the tree wilting. It can take some trial and error to figure out how often to water in your area. I assume it will spend most of its life indoors in your zone but when the temps are above 55 degrees bring your tree outside and let it grow there until the temps fall, it will be much happier.

I've come across a number of red oaks around here that look just like that. It's difficult to diagnose because the symptoms can be caused by a number of factors including drought, excess water/poor drainage, soil compaction, root damage, high salt levels in the soil, or bacterial leaf scorch. See oak leaf scorch . With the additional photos I'd agree that it's not likely to be iron chlorosis, though soil pH could be a contributing factor. It does not appear to be oak wilt.